There is a great deal of interest in “flat panel” displays, particularly for small to midsized displays, such as may be used in laptop computers, cell phones, and personal digital assistants. Liquid crystal displays (LCDs) are a well-known example of such flat panel video displays, and employ a matrix of “pixels” which selectably block or transmit light. LCDs do not provide their own light; rather, the light is provided from an independent source. Moreover, LCDs are operated by an applied voltage, rather than by current. Luminescent displays are an alternative to LCD displays. Luminescent displays produce their own light, and hence do not require an independent light source. They typically include a matrix of elements which luminesce when excited by current flow. A common luminescent device for such displays is a light emitting diode (LED).
LED arrays produce their own light in response to current flowing through the individual elements of the array. The current flow may be induced by either a voltage source or a current source. A variety of different LED-like luminescent sources have been used for such displays. The embodiments described herein utilize organic electroluminescent materials in OLEDs (organic light emitting diodes), which include polymer OLEDs (PLEDs) and small-molecule OLEDs, each of which is distinguished by the molecular structure of their color and light producing material as well as by their manufacturing processes. Electrically, these devices look like diodes with forward “on” voltage drops ranging from 2 volts (V) to 20 V depending on the type of OLED material used, the OLED aging, the magnitude of current flowing through the device, temperature, and other parameters. Unlike LCDs, OLEDs are current driven devices; however, they may be similarly arranged in a 2 dimensional array (matrix) of elements to form a display.
OLED displays can be either passive-matrix or active-matrix. Active-matrix OLED displays use current control circuits integrated with the display itself, with one control circuit corresponding to each individual element on the substrate, to create high-resolution color graphics with a high refresh rate. Passive-matrix OLED displays are easier to build than active-matrix displays, because their current control circuitry is implemented external to the display. This allows the display manufacturing process to be significantly simplified.
FIG. 1A is an exploded view of a typical physical structure of such a passive-matrix display 100 of OLEDs. A layer 110 having a representative series of rows, such as parallel conductors 111–118, is disposed on one side of a sheet of light emitting polymer, or other emissive material, 120. A representative series of columns are shown as parallel transparent conductors 131–138, which are disposed on the other side of sheet 120, adjacent to a glass plate 140. FIG. 1B is a cross-section of the display 100, and shows a drive voltage V applied between a row 111 and a column 134. A portion of the sheet 120 disposed between the row 111 and the column 134 forms an element 150 which behaves like an LED. The potential developed across this LED causes current flow, so the LED emits light 170. Since the emitted light 170 must pass through the column conductor 134, such column conductors are transparent. Most such transparent conductors have relatively high resistance compared with the row conductors 111–118, which may be formed from opaque materials, such as copper, having a low resistivity.
This structure results in a matrix of devices, one device formed at each point where a row overlies a column. There will generally be M×N devices in a matrix having M rows and N columns. Typical devices function like light emitting diodes (LEDs), which conduct current and luminesce when voltage of one polarity is imposed across them, and block current when voltage of the opposite polarity is applied. Exactly one device is common to both a particular row and a particular column, so to control these individual LED devices located at the matrix junctions it is useful to have two distinct driver circuits, one to drive the columns and one to drive the rows. It is conventional to sequentially scan the rows (conventionally connected to device cathodes) with a driver switch to a known voltage such as ground, and to provide another driver, which may be a current source, to drive the columns (which are conventionally connected to device anodes).
FIG. 2 is referenced to describe such a conventional arrangement for driving a display having M rows and N columns. A column driver device 260 includes one column drive circuit (e.g. 292, 294, 266) for each column. The column driver circuit 294 shows some of the details which are typically provided in each column driver, including a current source 284 and a switch 264 which enables a column connection 274 to be connected to either the current source 284 to illuminate the selected diode, or to ground to turn off the selected diode. A scan circuit 250 includes representations of row driver switches (208, 218, 228, 238 and 248). A luminescent display 280 represents a display having M rows and N columns, though only five representative rows and three representative columns are drawn.
Physically, the rows represented in FIG. 2 typically consist of a series of parallel connection lines traversing the back of a polymer, organic or other luminescent sheet, while the represented columns are typically constructed as a second series of connection lines, perpendicular to the rows and traversing the front of the luminescent sheet, as shown in FIG. 1A. Luminescent elements are established at each region where a row and a column overlie each other and thus form connections defining opposing sides of the element. FIG. 2 shows that each element as includes both an LED aspect (diode symbol) and a parasitic capacitor aspect (capacitor symbol “CP”).
In operation, information is displayed row by row during successive row scan periods. During a row scan period, each column connected to an element of the row which is intended to emit light is driven. For example, in FIG. 2 a row switch 228 grounds the row to which the cathodes of elements 222, 224 and 226 are connected during a scan of Row K. The column driver switch 264 connects the column connection 274 to the current source 284, such that the element 224 is provided with current. Each of the other columns 1 to N may be concurrently providing current to the respective elements connected to Row K, such as the elements 222 or 226. All current sources are typically at about the same amplitude, and OLED element light output is controlled by varying the amount of time during which the element conducts. When an OLED element has completed outputting light, its anode is pulled low to turn off the element. At the end of the scan period for Row K, the row switch 228 will typically disconnect Row K from ground and apply Vdd instead. Then, the scan of the next row will begin, with row switch 238 connecting the row to ground, and the appropriate column drivers supplying current to the desired elements, e.g. 232, 234 and/or 236.
Only one element (e.g. element 224) of a particular column (e.g. column J) is connected to each row (e.g. Row K), and hence only that element of the column is connected to both the particular column drive (294) and row drive (228) so as to conduct current and luminesce (or be “exposed”) during the scan of that row. However, each of the other devices on that particular column (e.g. elements 204, 214, 234 and 244 as shown, but typically including many other devices) are connected by the driver for their respective row (208, 218, 238 and 248 respectively) to a voltage source, Vdd. Therefore, the parasitic capacitance of each of the devices of the column is effectively in parallel with, or added to, the capacitance of the element being driven. The combined parasitic capacitance of the column limits the slew rate of a current drive such as drive 284 of column J. Yet rapid driving of the elements is necessary, as all rows must be scanned many times per second to obtain a reasonable visual appearance, allowing very little conduction time for each row scan. Low slew rates may cause large exposure errors, particularly for short exposure periods. Thus, for practical implementations of display drivers using the prior art scheme, the parasitic capacitance of the columns may severely limit drive accuracy.
A luminescent device matrix and drive system as shown in FIG. 2 is described, for example, in U.S. Pat. No. 5,844,368 (Okuda et al.). To mitigate effects of parasitic capacitances, Okuda suggests, for example, resetting each element between scans by applying either ground or Vcc (10V) to both sides of each element at the end of each exposure period. To initiate scanning a row, Okuda suggests conventionally connecting all unscanned rows to Vcc, and grounding the scanned row. The connection of all unscanned rows to a supply voltage, such as Vcc, effectively places the parasitic capacitance of all unscanned elements of the column line in parallel to the driven element. As such, changes in voltage on the driven element must also adjust the voltage on the combined column parasitic capacitance. The Okuda patent does not reveal any means to establish the correct drive level for a selected element at the moment of turn-on. The drive level required for display elements at a given current will vary as a function of display manufacturing variations, display aging and ambient temperature, and Okuda also fails to provide any means to compensate for such variation.
In view of the above, it may be appreciated that there is a need for a precharge process to reduce the substantial errors in OLED current which may result from employing a current drive for rapid scanning of OLED devices in a matrix having a large parasitic capacitance. Moreover, since the voltage for an OLED varies substantially with temperature, process, and display aging, a need may also be appreciated to monitor the drive levels of the OLEDs and to change the precharge process accordingly. Thus, what is needed in this industry is a means to adaptively apply correct precharging for scans of current-driven devices in an array.